Find some polarising sunglasses. You need at least two pairs (or take the lenses out of one so you have two separate polaroids). Place one lens on top of the other.
Put them on an overhead projector and rotate one of the pieces. What do you notice?
Now place a piece of sticky tape between the lenses. What happens? Try with other materials such as clear packaging from drink bottles, sweet wrappers or empty CD cases. Try twisting or stretching the plastic. Can you make an invisible picture out of the materials you have found?
Normal light is made from electromagnetic waves that vibrate in different directions as they move forward. Light can be polarised using a filter called Polaroid, which only lets light through that vibrates in one particular direction.
When two sheets of Polaroid are ‘crossed’ (see diagram) no light can get through at all. This happens because the first filter only lets through horizontally vibrating lightwaves, which then cannot get through the vertical ‘slots’ in the second filter. If one of the filters is turned by 90˚ the ‘slots’ run in the same direction and light can again get through.